Magnetic recording media are widely used for recording tape, video tape, computer tape, disc, etc. The recording density of magnetic recording media have become higher and higher, and their recording wavelength has become shorter and shorter every year. As the recording system for such the magnetic recording media there have been studied analog system as well as digital system.
The so-called coating type magnetic recording medium which comprises a dispersion of a ferromagnetic powder in a binder coated on a support to meet the requirement for higher density has a low packing density of magnetic material and deteriorated electromagnetic characteristics than the thin metal film type magnetic recording medium. However, the coating type magnetic recording medium can exhibit almost the same properties as that of the thin metal film type magnetic recording medium because of the recent development of magnetic materials and rapid progress of the technique for coating an extremely thin layer. Further, the coating type magnetic recording medium is excellent from the standpoint of practical reliability such as productivity and corrosion resistance.
As such the coating type magnetic recording medium there has been widely used a magnetic recording medium comprising a magnetic layer having a ferromagnetic iron oxide, Co-modified ferromagnetic iron oxide, CrO.sub.2, ferromagnetic alloy powder or the like dispersed in a binder coated on a non-magnetic support.
The enhancement of the electromagnetic characteristics of the coating type magnetic recording medium may be accomplished, e.g., by improvement of the magnetic properties of the ferromagnetic powder, smoothening of the surface thereof, etc. Various approaches have been proposed but leave something to be desired for the demand for higher density. The recent trend is for shorter recording wavelength with higher density. A problem of self demagnetization loss during recording or thickness loss during reproduction causing output drop is generated if the magnetic layer is too thick. Thus, an extremely thin coating type magnetic recording medium has been proposed.
In recent years, a so-called ME (metal-evaporated) tape on which a thin metal has been vapor-deposited (vacuum-evaporated) has been put into practical use in the field of Hi-8 and digital VCR for consumers' use. Thus, a system which employs both an alloy powder tape, i.e., so-called MP (metal particulate) and such an ME tape has been put into practical use.
In order to render MP tape compatible with ME tape, MP tape must have a magnetic layer with a reduced thickness to provide a higher output as in ME tape. Further, MP tape must have the same relationship between recording current and reproduced output as that in ME tape. Heretofore, as the recording current increases, MP tape gives a lower reproduced output due to recording demagnetization. However, ME tape doesn't show this tendency. ME tape shows a tendency for saturation of reproduced output with increase of the recording current. Therefore, Hi-8 decks actually employ a system in which recording is performed with different currents between MP tape and ME tape. However, this system is disadvantageous in that it requires a complicated circuit. In order to eliminate this difficulty, it is necessary that the same recording current be used in the system in which MP tape and ME tape are compatible with each other. However, there arises a difficulty that when recording and reproduction are performed on MP tape with a recording current optimum for ME tape, the resulting output is lowered. Further, when recording and reproduction are performed on ME tape with a recording current optimum for MP tape, ME tape cannot make the best use of its capacity, lowering the resulting output. It has thus been desired to make the optimum recording current for MP tape almost equal to that for ME tape.
In digital VCR's for consumers' use, a signal having a recording wavelength of 22 .mu.m has been used as a synchronization signal, and a signal having a recording wavelength of 0.488 .mu.m has been used for data recording. Further, in order to reduce the weight of the apparatus, the erasing head has been omitted, and an overwriting erasion system has been employed. In order to employ the overwriting erasion system, it is necessary that the synchronization signal be erased by the data signal. It is said that the overwriting erasure is preferably not more than -20 dB. It is preferred that the overwriting erasure be as low as possible to meet the requirements for magnetic recording medium. The thinner the magnetic layer is, the better are the overwriting characteristics. However, if the thickness of the magnetic layer is simply reduced, the total magnetization amount is reduced, thereby lowering the reproduced output of a long wavelength signal such as synchronization signal. In order to enhance the output of long wavelength signal, a large magnetization amount is needed. To this end, a high magnetic flux density and a certain thickness must be assured. As mentioned above, the overwrite characteristics and the output of long wavelength signal conflict with each other. However, the enhancement of the overwriting erasure can be accomplished not only by reducing the thickness of the magnetic layer but also by recording a short wavelength signal such as data signal deep into the magnetic layer. To this end, it is effective to allow the passage of as much recording current as possible upon the recording of data signal. As previously mentioned, in order to make MP tape compatible with ME tape, it is desirable that the optimum recording current for MP tape be as much as that for ME tape. This is desirable also from the standpoint of overwrite characteristics.
Besides the above-mentioned factors for the enhancement of overwrite characteristics, it is desirable that the surface of the magnetic layer be as smooth as possible so far as the running durability permits. It has been found that the higher the squareness ratio of the magnetic layer is, or the lower SFD (viz., Switching Field Distribution) is, the better is the overwrite characteristics thereof.
In order to improve the running durability, it has been a common practice to properly select a binder resin to be incorporated in the magnetic layer.
By way of example, a polyurethane which can form a rigid film having a high glass transition temperature (Tg) has been desirable to impart good durability to the magnetic layer. However, a polyurethane which form a film having a relatively low Tg has been selected from the standpoint of calenderability (i.e., improvement of performance in treatment for smoothing the surface of magnetic layer in addition to increasing the density of the magnetic layer, which is obtained by passing the magnetic tape at least one time through nip of a pair (nip) roll in which one of the rolls is a metal roll so that the magnetic layer comes in contact with the metal roll).
On the other hand, a magnetic recording medium which comprises a thinner magnetic layer provided on a non-magnetic layer has been known. However, in order to attain a higher recording density, it is necessary that the magnetic layer be thinner and the ferromagnetic metal powder be finer. The use of a finer ferromagnetic metal powder causes a drop of dispersibility and thus deteriorates the surface properties of the magnetic layer, thereby deteriorating the electromagnetic characteristics of the magnetic recording medium. Further, if the thickness of the magnetic layer is reduced, it is made difficult to secure sufficient durability.
For magnetic recording medium for digital VCR, a thin film head or MR head (viz., magnetoresistive reproducing head) is considered to be desirable. However, such the tape, if it comprises a vinyl chloride resin which has been widely used as a binder for magnetic recording medium, undergoes dehydrochlorination to release hydrochloric acid corroding the magnetic head even in a slight amount to cause troubles. Accordingly, a binder resin free of vinyl chloride has been desired.
The applicants have proposed an extremely thin layer type magnetic recording medium which comprises a thinner magnetic layer as an upper layer provided on a non-magnetic layer as a lower layer. Examples of these inventions will be given below.
U.S. Pat. No. 4,863,793 discloses a magnetic recording medium comprising a dispersion of a ferromagnetic powder having an average long axis length of less than 0.8 .mu.m and a crystallite size of less than 300 .ANG. coated on a non-magnetic layer.
U.S. Pat. No. 5,496,607 discloses a technique which comprises the definition of the thickness of the magnetic layer to less than 0.3 .mu.m and the standard deviation of the thickness of the magnetic layer to a predetermined range to obtain a magnetic recording medium having a good overwrite characteristics and a less strain during digital recording.
However, these conventional techniques generally employ a polyurethane and a vinyl chloride resin as binder resin and thus leave something to be desired in the enhancement of overwrite characteristics and the assurance of output at low frequencies. Further, these conventional techniques can hardly meet the requirements of both running durability and prevention of head corrosion. In other words, sufficient running durability cannot be assured without any vinyl chloride resin. On the contrary, if any vinyl chloride resin is used, head corrosion cannot be inhibited. In particular, the conventional techniques leave something to be desired in overwrite characteristics and compatibility with ME tape in 24-25 modulation employed for digital VCR for consumers' use and running durability and corrosion resistance. Further, no binder resin systems free of vinyl chloride resin which can solve the foregoing problems have been found.